sqlplus登录提示：ORA-12162：TNS:netservicenameisincorrectlyspecified错误 - 数据库编程

[root@localhost ~]# su - oracle
[oracle@localhost ~]$ sqlplus '/as sysdba'

SQL*Plus: Release 11.2.0.1.0 Production on Tue Apr 22 17:10:52 2014

Copyright (c) 1982, 2009, Oracle. All rights reserved.

ERROR:
ORA-12162: TNS:net service name is incorrectly specified

Enter user-name:
ERROR:
ORA-12162: TNS:net service name is incorrectly specified

Enter user-name:
ERROR:
ORA-12162: TNS:net service name is incorrectly specified

SP2-0157: unable to CONNECT to ORACLE after 3 attempts, exiting SQL*Plus
[oracle@localhost ~]$ export ORACLE_SID=PROD //向环境变量中添加实例名，说明需手动设置
[oracle@localhost ~]$ sqlplus '/as sysdba'

SQL*Plus: Release 11.2.0.1.0 Production on Tue Apr 22 17:11:16 2014

Copyright (c) 1982, 2009, Oracle. All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> select status,instance_name from v$instance; //登录后查询状态,正常

STATUS INSTANCE_NAME
------------ ----------------
OPEN PROD